Long-term treatment with streptozocin/5-fluorouracil chemotherapy in patients with metastatic pancreatic neuroendocrine tumors

Abstract Rationale: Pancreatic neuroendocrine tumors (pNETs) are rare entities representing 1% to 3% of all malignant pancreatic neoplasms. Current guidelines recommend a combination of streptozocin (STZ) and 5-fluorouracil (5-FU) for patients with metastatic well-differentiated pNETs requiring systemic therapy. The highest median progression-free survival rate reported in previous studies for this combination was 23 months (95% confidence interval 14.5–31.5). However, it remains unclear for how long this regimen can be safely administered. Patient concerns: We report about 3 therapy-naïve patients with metastatic G2 (Ki67 10%–15%) pNETs treated with STZ/5-FU, that achieved sustained disease control for longer than 36 months. Diagnosis: Metastatic, well-differentiated G2 pNETs Interventions: Systemic chemotherapy with STZ/5-FU was administered until the disease progressed. In 1 case showing a mixed response, selected metastases of increasing size were additionally treated with surgery and brachytherapy. Outcomes: In our 3 patients with metastatic G2 pNETs, STZ/5-FU induced long-term disease control over 44, 42, and 95 months, respectively. No side effects that led to treatment discontinuation were observed. Lessons: In patients with metastatic G2 pNETs achieving disease control, STZ/5-FU can be safely administered.


Introduction
Pancreatic neuroendocrine tumors (pNETs) are rare malignancies with an overall incidence of <1/100.000, [1] and they constitute only approximately 3% of all pancreatic neoplasms. [2] They account for a stable proportion of approximately 7% of all neuroendocrine tumors (NETs) over the last few decades. [3] pNETs commonly occur between the sixth and eighth decades of life, [4][5][6] and males are affected slightly more often than females. [4,5] Patients with genetic predispositions, such as multiple endocrine neoplasia type 1 and von Hippel-Lindau syndrome, have a significantly increased risk of developing pNETs and show an earlier onset of the disease. [7][8][9] In most cases, pNETs have an indolent clinical course and are first diagnosed when the disease is locally advanced or metastatic. [10][11][12] pNETs are characterized by a very heterogeneous biology and can present either with symptomatic hormone production (gastrinoma, insulinoma, glucagonoma, VIPoma), [13][14][15][16] or with no hormone production. [17] Prognostic factors for patients with pNETs include age, performance status, tumor stage, serum chromogranin A levels, and the tumor proliferation marker Ki-67, whereas predictive markers are still lacking. [18][19][20][21][22][23] Currently, surgical resection is the only curative therapeutic approach. [24] For patients with unresectable disease, numerous therapies are available, including interferon, somatostatin analogs, chemotherapy with streptozocin (STZ) plus 5-fluorouracil (5-FU) or temozolomide plus capecitabine, targeted therapies with everolimus or sunitinib, and peptide receptor radionuclide therapy. [24][25][26][27][28][29][30] According to current guidelines, systemic chemotherapy with STZ/5-FU is the standard first-line therapy to induce disease control in patients with advanced pNETs not amenable to resection. [31][32][33] This recommendation is based on the trials of Moertel et al, [34,35] who achieved remission in 43% to 63%, as well as an overall survival of 26 to 42 months in patients with metastatic or locally advanced pNETs treated with STZ/5-FU or STZ/doxorubicin. However, it is unclear how long these regimens can be safely administered and how long remission can be maintained. Here, we present 3 patients with pNETs in whom STZ/5-FU induced long-term disease control and was safely administered over the years.

Case reports
A detailed summary of patient characteristics, tumor-specific data, and clinical course data is presented in Table 1.

Case 1
In August 2014, a 77-year-old Caucasian man presented to our department for diagnostic workup of liver and pancreatic lesions. In 2010, he was diagnosed with prostate cancer (stage 2b, Gleason 3+4 = 7a), which was treated with curative radiation to the prostate and seminal vesicles. During restaging examinations by computed tomography (CT), new lesions within the liver and pancreatic corpus were detected ( Fig. 1A and B). Histological examination of the liver biopsy specimen revealed a welldifferentiated NET, G2, with a Ki-67 index of 10%. Endoscopic examination excluded a primary tumor outside the pancreas. No pulmonary metastases were observed on the CT of the thorax. In summary, we diagnosed the patient with a hepatic metastatic pNET. The patient received systemic chemotherapy using the STZ/5-FU regimen described by Moertel et al [34] (STZ 500 mg/m 2 and 5-FU 400 mg/m 2 , days 1-5, qd 43) starting in September 2014. After 4 cycles of chemotherapy, the patient achieved partial remission ( Fig. 1 C and D). Overall, he tolerated the therapy well during the first 18 cycles (September 2014 to September 2016), without relevant clinical or laboratory side effects. However, after the 18th cycle, he developed urosepsis with acute renal Table 1 Summary of patient characteristics, tumor specific data, and treatment data of the 3 presented cases.  (April 2017) with persistent remission, we switched to the Uppsala regimen (STZ 1000 mg/m 2 and 5-FU 400 mg/m 2 , d1, qd 22). The patient tolerated this regimen well without relevant clinical side effects or stable retention values for an additional 20 cycles. In total, he received 41 cycles of STZ/5-FU-based therapy over a period of 44 months, with sustained remission. In June 2018, an increase in glomerular filtration rate was observed, and chemotherapy was stopped. The chronic renal failure was the consequence of an obstructive uropathy linked to the presence of a phimosis, a prostate hypertrophy and the previous radiotherapy. The additional etiological contribution of previously administered STZ-based chemotherapy to chronic kidney failure cannot be excluded. After further deterioration of the glomerular filtration rate despite medical and urological therapy, hemodialysis was initiated. The patient died shortly thereafter in August 2018 due to dialysis catheter-associated sepsis.

Case 2
A 52-year-old Caucasian man presented to our department in September 2016 with further histological clarification of a liver lesion. He was diagnosed with multiple endocrine neoplasia type 1 (MEN 1), which was responsible for multiple surgeries for parathyroid adenomas in 1990, 1992, and 2004. Because of unclear hepatic lesions detected on a CT scan of the abdomen, we performed a Ga68 DOTANOC positron emission tomography/ computer tomography (PET/CT), demonstrating multifocal pathological uptake in the pancreatic tail, both liver lobes, as well as in the thoracic and lumbar spine and the os ilium ( Fig. 2A  and B). An ultrasound-guided liver biopsy was performed.  (Fig. 2C). STZ/5-FU according to Uppsala was continued over a period of 42 months (56 cycles) until disease progression was assessed by Ga-68-DOTANOC PET/CT in May 2020 (Fig. 2D). STZ/5-FU therapy was well tolerated, and no therapy-limiting clinical or laboratory adverse events occurred. Further therapies included everolimus 10 mg daily and peptide radio-receptor therapy, which was started in January 2021.

Case 3
Our third case involved a 56-year-old woman who presented to our department for the initiation of systemic chemotherapy. In October 2009, the patient presented with non-specific upper abdominal complaints. Abdominal CT revealed an inhomogeneous mass in the pancreatic tail with infiltration of the spleen and transverse colon (Fig. 3A). Ultrasound-guided biopsy of the   (Fig. 3B). However, no radionuclide accumulation was detected on somatostatin receptor (SSR) scintigraphy. Based on these results, she received systemic chemotherapy with the STZ/5-FU protocol according to Moertel et al [34] (STZ 500 mg/m 2 and 5-FU 400 mg/m 2 d1-5, qd 43). Partial remission was achieved after 2 cycles. STZ/5-FU was further administered, and the disease control lasted until September 2015. During this period, chemotherapy was well tolerated, no dose reduction or therapy delay was necessary, and no relevant clinical or laboratory side effects were observed. After 22 cycles (January 2013 to August 2015), new lesions in the ileocecal area and ascending colon were observed on a CT scan of the abdomen in September 2015 ( Fig. 3C and D), whereas liver metastases were still in remission. These lesions showed additional SSR positivity in the Ga-68-DOTANOC PET/CT performed in October 2015. She underwent debulking surgery with resection of the progressive lesions (omentum majus and uterine tumor). In the resected specimen, a moderately differentiated NET was confirmed, with a Ki-67 index of 20%, which was higher than that of the initial histology. After reconvalescence, systemic therapy with STZ/5-FU was resumed, first according to the Moertel protocol (January 2016 to February 2017), followed by the Uppsala regimen, again with good tolerability and sustained remission for a further 31 months. In September 2018, Ga-68-DOTANOC PET/CT showed a mixed response, with an increase in the size of 2 SSR-positive hepatic metastases in segments I and V/VI. After discussion of possible therapeutic options for the patient, the progressive hepatic lesions were treated with CT-guided interstitial brachytherapy with an Iridium-192 source in November 2018, whereas STZ/5-FU chemotherapy according to the Uppsala regimen was further administered until January 2020. During this period, only an SSR-positive lesion in the pancreatic head increased in size, whereas the remaining NET manifestations were in remission. Therefore, a pancreatic head resection was performed in April 2020. Histopathological examination revealed a poorly differentiated NET, with a Ki-67 index of 35%. After recovery, chemotherapy with STZ/5-FU was resumed, with sustained remission. To date, the patient has received 86 cycles of STZ/5-FU over a period of 95 months.

Discussion
Herein, we report 3 therapy-naïve patients with metastatic, hormone-inactive pNETs who achieved sustained remission of all or a proportion of tumor manifestations under treatment with STZ/5-FU. All patients were initially treated according to the Moertel protocol and were switched on to the more patientfriendly Uppsala protocol. [32] In 3 cases, partial remission was achieved after 3, 5, and 8 months. Our patients received a median of 56 cycles over a median treatment period of 44 months, with a median progression-free survival of 44.3 months and a median overall survival (mOS) of 59 months. A similar mOS (54.8 months) was reported by Dilz et al [36] in a German retrospective study of 96 pNET patients who were treated according to the Moertel regimen. However, specific data on the subgroups of patients with pNET with different grades (G1, G2, and G3) are missing in the publication by Dilz et al, which prevented us from making a direct comparison with our results.
Similar to our data, in a retrospective analysis by Antonodimitrakis et al [37] including 133 patients with pNETs of different grades who received STZ/5-FU according to the Uppsala regimen, 14% of the patients received chemotherapy for at least 60 months and 4% for as long as 120 months. In particular, in this study, the authors increased the interval between STZ/5-FU courses in responding patients to allow long-term treatment. A similar strategy was reported in a German study on patients with pNETs of different grades by Schrader et al, [38] where an extended cycle length (3 months) was administered as maintenance therapy after achieving disease control, which was associated with an mOS as long as 69 months. Whether patients achieving disease control with STZ/5-FU may have a higher benefit in terms of survival, safety, and quality of life by continuing the therapy (as in our patients), by increasing the interval between STZ/5-FU courses, as in the above-mentioned studies, or by stopping the therapy is not known. Furthermore, we switched the regimen from induction therapy to the more patient-friendly Uppsala protocol after achieving remission, which may have had a beneficial effect on the quality of life of our patients.
In line with the aforementioned retrospective analyses, longterm STZ/5-FU treatment was safe. [36][37][38] Indeed, grade 3/4 toxicities leading to dose reduction or discontinuation are rare in patients treated with STZ/5-FU, even over the years. The most Medicine common adverse events observed in patients receiving STZ/5-FU were fatigue, diarrhea, stomatitis, nausea, thrombocytopenia, leukopenia, and impaired renal function. [35][36][37]39,40] The slightly decreased glomerular filtration rate described in case 1 was observed after a uroseptic event and was therefore judged to be unrelated to treatment. In all 3 cases, we did not observe treatment-related toxicities that led to dose adjustment, an increase in the interval between STZ/5-FU courses, or treatment discontinuation.

Conclusion
In the case series presented here, we summarized 3 therapy-naïve patients with metastatic G2 pNETs who received long-term treatment with STZ/5-FU for >3 years. Long-term STZ/5-FU therapy is feasible and safe for therapy-naïve patients with metastatic G2 pNETs. Thus, for patients achieving disease control, long-term STZ/5-FU therapy may represent a valuable therapeutic option to achieve long-lasting remission with good tolerability. Prospective trials may further define the optimal maintenance strategy for this selected population.

Author contributions
CM developed the original idea, drafted the manuscript and tables, and selected figures for the reported cases. MCK, SK, AK, VK, and MV provided critical feedback, helped shape the manuscript, and were responsible for the important intellectual content. MV conceived the original idea and supervised the project. All authors issued final approval for the version to be submitted.